The present invention relates to an ultrasonic generator which may be used in particular in dentistry.
Heretofore known ultrasonic generators generally comprise power transducers of the piezoelectric or magnetostrictive type, and they may be classed in two distinct categories depending on whether the transducer used is tuned to the series frequency or the parallel frequency.
A transducer operating on its parallel frequency is characterised in that, on the one hand, its impedance attains its maximum for the parallel frequency and, on the other hand, an increase in the mechanical load provokes a reduction in the impedance.
Certain constructors use this solution as it enables a good adaptation of power in load to be obtained by means of a voltage supply. However, the principal drawback of the circuits using a transducer tuned to the parallel frequency is that it is necessary to apply on the transducer very high voltages even for applications of moderate power. This renders this solution unsuitable for medical applications or for very high power industrial applications.
The functioning of a transducer tuned to the series frequency is the opposite of that of a transducer tuned to the parallel frequency. In fact, a transducer functioning on its series frequency is characterised in that, on the one hand, its impedance attains its minimum for the series frequency and, on the other hand, an increase in the mechanical load provokes an increase in the impedance.
There again, numerous known apparatus use, for the transducer tuned to the series frequency, a variable voltage supply for controlling the ultrasonic oscillator. However, this solution has the drawback of causing the transducer to operate irrationally and dangerously. In fact, the consequence of this type of supply is that the power furnished to the transducer decreases, as a function of the impedance of said latter, according to a hyperbolic law and therefore the greater the mechenical work required, the less the generator furnishes power, which is totally illogical. What is most serious with such a solution, is that, when the transducer is no longer charged, the power which is applied thereto tends towards the infinite, which, in practice, may be translated by a breakdown of the ultrasonic tool.
It is seen from the above that the voltage supply systematically opposes the functioning of a transducer tuned to its series frequency.
Attemps have already been made to remedy this drawback by using complex massive circuits tuned to the series frequency of the transducer. However, these solutions present in turn the drawback of leading to bulky circuits, of requiring very precise passive components, which are therefore not easily reproducible, and of being able to function only on one frequency.